El Pluscuamperfecto Subjuntivo in hypothetical clauses (si): impossible condition

One of the examples is If we had won the lottery, we would have bought a Ferrari.

The pluscuamperfecto subjuntivo It is used when we speak of an unlikely or hypothetical condition [If they had won the lottery] However, they didn't win it and therefore they could not buy a Ferrari. 

It's impossible, because it's in the past and we already know the result. If the lottery has just been run, but we haven't checked our ticket yet, then it's unlikely we've won, but not impossible and you wouldn't use the pluscuamperfecto subjuntivo.

"If you had taken the umbrella, you wouldn't have got so wet."  Taking an umbrella is not intrinsically unlikely, but we know it didn't happen so it's now impossible.